City Living Can Drive You Crazy

Action Points

Explain to interested patients that increased social fragmentation in large cities may partially explain why psychotic disorders are more common in urban areas.

Note that the study was limited by a lack of data on potentially important factors such as cannabis use, reliance on administrative records for data, and some of the data used in the study was relatively crude.

Increased social fragmentation in large cities may partially explain why psychotic disorders are more common in urban areas, researchers have suggested.

Results from a long-term study of more than 200,000 people in Sweden indicated that area-level variables largely explained the correlation between urban residence and increased prevalence of schizophrenia, reported Stanley Zammit, PhD, of Cardiff University in Wales, and colleagues.

Residence in a city was associated with a 41% greater likelihood of psychosis compared with rural residents (odds ratio 1.41, 95% CI 1.09 to 1.82), they wrote in the September issue of Archives of General Psychiatry.

Among the area-level variables examined, social fragmentation at the school level -- reflected in the proportion of children who were immigrants, changed cities between the ages of 8 and 16, or were raised in a single-parent household -- were the most important, they added.

"Our findings highlight the concern that physical integration [in social settings] alone is not sufficient, but that some of the positive characteristics traditionally conferred by segregation, such as a localized sense of safety, cohesion, and community spirit, must also be maintained to enhance the mental health of individuals within the population," they wrote.

Previous studies of variations in prevalence of psychotic disorders have found substantially higher rates, per 1,000 population, in cities relative to rural areas. However, as Zammit and colleagues noted, those studies could not determine whether cities act as magnets for people with psychotic tendencies or whether something about the urban environment promotes psychosis.

The new study comes down on the side of the latter.

The study sample was a cohort of Swedish residents born from 1972 to 1977 and followed through 2003 for diagnoses of schizophrenia or other nonaffective psychoses recorded in the country's national health registry.

"The association between living in a city and psychosis was attenuated, but nevertheless persisted, when adjustment was made for individual-level factors," Zammit and colleagues wrote. The city-rural odds ratio dropped only to 1.32 (95% CI 1.01 to 1.72) after controlling for such individual characteristics as gender, family history of schizophrenia, immigrant status, and single-parent family.

On the other hand, controlling for school- and municipality-level variables mostly eliminated the relationship between "urbanicity" and psychosis prevalence (OR 1.10, 95% CI 0.77 to 1.57).

At the school level, these variables included social fragmentation and the percentages of the population that was foreign-born or economically deprived. Municipal-level variables were social fragmentation, deprivation, and population density.

The only one among them that was significantly associated with psychosis prevalence by itself was school-level social fragmentation (adjusted OR 1.09, 95% CI 1.01 to 1.18).

But for some of these variables, there were significant interactions between individual and higher-level variables.

For example, Zammit and colleagues wrote, "individuals who were foreign-born were at a high risk of developing psychosis if they were part of a school group with very few others who were foreign-born, and this risk decreased if their school group consisted of a large proportion of foreign-born individuals."

They scored individuals for social fragmentation, with one point for each of the three possible components. Statistically, for each point of individual social fragmentation, a 10% increase in the average for that individual's school decreased the risk of psychosis by 8% (P=0.004).

In other words, similarity of background between individuals and their school-level peer groups diminishes the risk of psychosis.

Limitations to the study included lack of data on potentially important factors such as cannabis use, and some of the data used in the study was relatively crude. For example, Zammit and colleagues noted that area-level social fragmentation and deprivation are "difficult to measure." The reliance on administrative records for all study data was also a limitation.

The study was funded by the National Assembly for Wales and Swedish Research Council for Working Life and Social Research.

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